Alumina trihydrate or Al.sub.2 O.sub.3.3H.sub.2 O is generally produced by the well-known Bayer process and its main area of application is in the production of metallic aluminum by electrolytic reduction. Alumina trihydrate, due to its considerable water content, cannot be directly utilized in electrolytic reduction cells. It has to be subjected to a calcination step prior to its use as feed to reduction facilities.
The Bayer process, as it is known, involves the digestion of bauxite in a caustic medium at elevated temperatures and pressures and from the resulting caustic liquor, the dissolved alumina content is recovered by subjecting the liquor to precipitation. Precipitation is generally accomplished by the use of alumina trihydrate seed and the recovered alumina trihydrate after several process stages, such as classification, filtration, and washing, is subjected to a calcination step to remove the major portion of its water content prior to its use as reduction-grade alumina.
It is also known that there are two variations to the Bayer process, the American and the European processes, and these two processes produce alumina trihydrate of varying particle size distribution and strength. While the American Bayer process generates coarse alumina hydrate particles, the European type process results in considerably finer hydrate grains. Calcination of these two types of hydrates also results in different products. For example, calcination of the hydrate produced by the American type Bayer process results in a relatively coarse alumina, often designated as "sandy" alumina; while the alumina obtained by the calcination of the hydrate generated by the European type Bayer process is a much finer product, characterized by a "floury" appearance and properties. When these hydrates are calcined in conventional calcination equipment, such as rotary kilns or fluidized bed furnaces, a relatively large quantity of dust will be generated due to thermal shock and/or attrition affecting both the American type coarse alumina hydrate and the European type hydrate, which hydrate is already finely divided. These fines, to avoid air pollution and product losses, must be retained in the calcination system. This is generally accomplished by the use of dust collectors, such as electrostatic precipitators or the like, and the recovered partially calcined alumina dust is commonly referred to as electrostatic or ESP dust.
The recovered alumina dust is generally characterized by a particle size distribution wherein particles of less than 44 microns in size dominate, most commonly 90% by weight or more of the alumina has a size below 44 microns. Additionally, the dust is a mixture of calcined, partially calcined and uncalcined particles and, consequently, the water content of the individual particles as determined by loss on ignition (LOI) test can vary between wide limits, for example, between about 1 and 35% by weight. These properties of the alumina dust render it a waste material and its disposal creates several problems since the dust can amount to about 5-10% relative to the total quantity of alumina recovered from the calciner. In the case of a calcining facility having an annual calcined alumina output of 500,000 metric tons per year, the alumina dust loss can amount to 25-50,000 tons per year. To at least reduce this significant by-product loss at certain calcination facilities, a portion of the generated alumina dust is blended with the calcined alumina product. Blending, however, cannot eliminate the problem since the calcined alumina product always contains particles of less than 44 micron size and the percentage of the less than 44 micron size alumina cannot be increased beyond a certain acceptable limit set by the operators of reduction facilities. Thus, only a portion, and generally only a small fraction, of alumina dust can be utilized since in addition to the particle size, the LOI of the dust can also detrimentally affect the quality of the calcined alumina. Another method of reducing the accumulated quantity of alumina dust is to recycle the dust to the digestion step where it is redissolved in caustic to form alumina trihydrate which is then recovered by precipitation. This method, while capable of handling the alumina dust problem, is economically disadvantageous since redissolution requires additional processing and the overall productivity of the Bayer plant is reduced in direct relationship with the quantity of redissolved alumina dust.
It has also been suggested in U.S. Pat. No. 4,051,222--Gnyra (Sept. 27, 1977) to coarsen alumina dust produced in calciners for reuse in the Bayer process. Coarsening or agglomeration of the dust by the process described in this patent is accomplished by admixture with Bayer process pregnant liquor and calcium carbonate in an aqueous medium. The produced slurry containing up to about 50 grams calcium carbonate per liter of pregnant liquor is used as seed for alumina trihydrate precipitation from pregnant Bayer process liquor. Although this prior art process allows the utilization of a portion of the partially calcined alumina dust, the employment of calcium carbonate as an agglomerating or coarsening agent generates significant problems. These problems originate from the introduction of a calcium-containing compound into the pregnant liquor. The compound under the precipitation conditions envisioned can form calcium aluminate. Additionally, the carbonate constituent of the calcium compound may react with the caustic content of the liquor resulting in the formation of undesired sodium carbonate.
In U.S. Pat. No. 4,311,486--Yamada et al (Jan. 19, 1982), alumina hydrate is precipitated from Bayer process pregnant liquor by adding as seed finely divided alumina hydrate having an average particle size of less than 10 microns. This fine alumina hydrate is recovered from precipitated alumina hydrate by classification or by addition of aluminum hydroxide gel to supersaturated sodium aluminate solution. This process is limited in its application since it can only employ alumina hydrate for the seeding operation and not the partially calcined dust, which hydrate must be obtained prior to calcination by classification. Also, since the average particle size recommended for seeding is about 10 microns or less, this requires an extensive separation process and no provision is made for the alumina hydrate falling within the 10-44 micron range. Consequently, this prior art process cannot be applied to the elimination of the alumina dust problem.
It has now been found that the quantity of partially calcined alumina dust which is generated in alumina hydrate calcination facilities can be significantly reduced. This is accomplished by using the partially calcined alumina for precipitating alumina hydrate from supersaturated Bayer process pregnant liquors. Precipitation is accomplished by using either the partially calcined alumina as seed under predetermined conditions or by admixing the partially calcined alumina with conventionally utilized alumina hydrate seed material in a predetermined ratio. The alumina hydrate which precipitates from the liquor seeded with the partially calcined alumina exhibits coarseness and strength and can be subjected to calcination without undue formation of unacceptable fines.